When FEC is executed as an error correction technique, an FEC frame storing therein an error correcting code is used for data transmission. As a conventional FEC-frame configuring method in an optical communication system, a method of expanding a redundant area of an OTUk (Optical channel Transport Unit-k) frame has been applied (see, for example, Patent Literature 1).
Assuming that the redundant area of the OTUk frame is expanded, it has been suggested that a redundancy of 20% is appropriate taking into consideration a trade-off relation between an increase of transmission penalty due to an increase in the transmission rate and coding gain due to expansion of the redundant area (see, for example, Non Patent Literature 1).
In a digital optical transmission system, a method of providing a plurality of kinds of error correction circuits to switch the error correction circuits according to a state of a transmission path has been proposed (see, for example, Patent Literature 2).